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IEEE Transactions on Applied Superconductivity

Lumped 50- Ω\OmegaΩ Arrays of SNS Josephson Junctions

N. Hadacek, Paul D. Dresselhaus, Samuel P. Benz

Published Dec 1, 2006• 6 citations
Publisher

Abstract

For the first time, lumped series arrays of Josephson junctions have been fabricated with a transmission line-matched 50-Omega resistance. These arrays also have the thousands of junctions necessary to produce a metrologically significant voltage. This approach is expected to increase the output voltage per array and to optimize their performance for Josephson voltage standards. Traditional Josephson arrays for voltage standards have used distributed microwave structures, where array lengths are several multiples of the driving wavelength. The lumped arrays in this work have physical lengths shorter than a quarter of the microwave drive wavelength and total normal-state resistances nearly equal to the transmission line impedance. Fabrication of these arrays was made possible by use of a newly developed Nb-(MoSi2-Nb)nstacked junction technology. We present measurements of the microwave response of lumped arrays with total normal resistances up to 54 Omega and with various termination resistances. A simple numerical model is presented that accounts for the spatial distribution of the microwave current and for the nonuniformity of the junction critical currents. The resulting simulations agree well with experimental results

DOI
10.1109/tasc.2006.883456

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For the first time, lumped series arrays of Josephson junctions have been fabricated with a transmission line-matched 50-Omega resistance. These arrays also have the thousands of junctions necessary to produce a metrologically significant voltage. This approach is expected to increase the output voltage per array and to optimize their performance for Josephson voltage standards. Traditional Josephson arrays for voltage standards have used distributed microwave structures, where array lengths are several multiples of the driving wavelength. The lumped arrays in this work have physical lengths shorter than a quarter of the microwave drive wavelength and total normal-state resistances nearly equal to the transmission line impedance. Fabrication of these arrays was made possible by use of a newly developed Nb-(MoSi2-Nb)nstacked junction technology. We present measurements of the microwave response of lumped arrays with total normal resistances up to 54 Omega and with various termination resistances. A simple numerical model is presented that accounts for the spatial distribution of the microwave current and for the nonuniformity of the junction critical currents. The resulting simulations agree well with experimental results
NIST is using and developing superconductor-normal metal-superconductor (SNS) Josephson arrays for both programmable DC and AC voltage standards. Increasing the output voltage is difficult because the output voltage per junction is small; hence series arrays with large numbers of junctions are needed. The best way to generate higher voltages and achieve the best operating margins for the broadband drive signals is by densely packing the junctions into shorter arrays. NIST has been working on stacked SNS junctions to achieve this goal. By stacking junctions in the array, more junctions may be placed per length, while preserving a lumped microwave element. In this paper we introduce our results on stacked SNS junctions using MoSi/sub 2/ and Ti as barrier materials. These barriers were chosen because they can be reactive-ion etched (RIE) in contrast to our standard PdAu barriers, which must be wet etched. Using RIE, alternating layers of barrier material and Nb may be etched in a single step. We indirectly quantify the junction uniformity in the arrays by measuring the current range of the constant-voltage steps when the arrays are biased with a microwave drive.
Superconductor/normal conductor/superconductor (SNS) Josephson junction series arrays are the subject of research for the realization of a Josephson arbitrary waveform synthesizer (JAWS) based on pulse-driven Josephson junctions. Lumped arrays of SNS Josephson junctions are investigated for this application at the Physikalisch-Technische Bundesanstalt (PTB). The series arrays consisting of Nb/HfTi/Nb SNS junctions are embedded in the center line of a coplanar waveguide transmission line. To increase the ac output voltage, the Josephson junction series array is designed as a meander-like structure. A broadband response of the arrays is required for the complex pulse operation of the JAWS. The arrays are investigated by dc measurements and for test purposes under continuous microwave irradiation at different frequencies. Flat Shapiro steps were measured for meander-like series arrays with 21 parallel lines containing up to 10 752 junctions. A sine wave with a frequency of 2.5 kHz and a peak voltage of 214 mV Peak was synthesized under pulse mode operation.
The Josephson arbitrary waveform synthesizer (JAWS) allows AC voltages to be generated with fundamental accuracy. Lumped arrays of superconductor/normal conductor/superconductor (SNS) Josephson junctions were developed and investigated for this application. Combining electron-beam lithography and chemical-mechanical polishing, series arrays of sub-micrometer SNS junctions consisting of Nb/HfTi/Nb were fabricated. The junction size varied from 0.04 mum2to 4 mum2in series arrays up to 2560 junctions in different microwave designs. Stable Shapiro steps are generated in a frequency range from 5 GHz to 20 GHz using continuous microwave irradiation. Using a simple pulse pattern at different pulse repetition frequencies, the arrays show current-voltage characteristics with flat Shapiro steps. Sinusoidal AC voltages were synthesized using a commercial pulse pattern generator. Peak-to-zero voltages of some mV for frequencies below 1 kHz were achieved. Higher harmonics were measured to be more than 80 dB below the fundamental tone. The behavior of the arrays was investigated in more detail by measuring the amplitude-frequency spectra.
We have applied our existing focused ion beam based nanoscale planar SNS bridge junction technology to the fabrication of series arrays, with a view to lumped array applications. Single junctions are created in a micrometre-width bilayer track (125 nm Nb on 75 nm Cu) by milling a trench 50 nm wide in the upper superconducting layer. Individual junctions with non-hysteretic, resistively-shunted current-voltage characteristics, critical current I C ~1 mA and characteristic voltage I C R N ~50 µV at 4.2 K can be fabricated routinely. The characteristics of 10-junction series arrays at junction spacings of 0.2 µm to 1.6 µm have been studied at 4.2 K and above. Locking of all the junctions in the array under an applied microwave field is observed as the transition temperature is approached. This effect is achieved at a lower temperature for shorter junction spacings, suggesting a penetration depth-dependent electromagnetic coupling mechanism. Measurements of differential resistance versus current reveal the I C distribution of individual junctions within the array. Spreads of critical current δ I C = I Cmax / I Cmin ~1.5 at 4.2 K are typically observed. The spread in normal state resistance, R N between junctions is negligible in comparison, as the unbroken normal metal layer shunts all of the junctions in the array. This allows locking to be achieved in spite of the appreciable spread in I C .
Programmable Josephson voltage standards for output voltages up to 10 V have been realized. Binary-divided series arrays consisting of 69 632 overdamped SINIS Josephson junctions have been fabricated using reliable Nb-Al/Al2O3technology (S: superconductor, I: insulator, N: normal metal). Series arrays of about 1000 SNS Josephson junctions were realized as lumped arrays for applications in the Josephson arbitrary waveform synthesizer (JAWS). We suggest a novel scheme for combining the higher voltages generated by the binary-divided arrays with the frequency spectral characteristics of the JAWS, in order to optimize the frequency spectrum of the generated ac voltages
Superconductor-normal metal-superconductor (SNS) junctions are well-suited for large-scale integrated superconducting circuits. Potential applications are programmable voltage standards, digital-to-analog converters and large-scale RSFQ circuits. The application of SNS junctions provides some advantages, e.g. a simplified fabrication technology and reduced parasitic inductances. But it requires a technological process with dimensions down to the submicrometer region to achieve sufficiently high characteristic voltages I/sub C/R/sub N/. In our process the normal metal interlayer is titanium. Therefore the sidewall insulation of the junctions can be made by anodization, as it is known from the Nb-Al technology. Nb-Ti-Nb Josephson junctions and junction arrays were fabricated down to the submicrometer range with I/sub C/R/sub N/ values up to 87 microvolt depending on the titanium thickness. Their measured critical currents have a small spread of about 10 percent. Shapiro steps are observed under the influence of microwave irradiation. For the application of such junctions in a programmable Josephson voltage standard a new type of microwave circuit using coplanar strips is proposed and successfully tested in 10 V SIS circuits.